Pleated and/or electrically charged fibrous materials to be used as a filtration medium have been known for some time. The electrostatic charging of fibrous materials and its shortcomings are reviewed in U.S. patent application No. 5,401,446, issued Mar. 28, 1995, and incorporated herein by reference.
Currently available methods for cold charging a web have problems developing the desired charge densities and, in addition, suffer from the problem of having the charge bleed off the web with time.
Pleating the filters increases filtering efficiency, without producing as high a pressure drop as would be caused by more closely packing the fibers. Pleating also results in increased ability to retain dust which is filtered and removed from the environment.
A need exists for pleated and non-pleated composite structures, useful for filters, which have the capability to maintain a high level of electrostatic charge.